1. Field of the Invention
The present invention relates to a turbo chiller and a control method therefor that achieve heating and cooling by means of a centrifugal compressor, which is turbomachinery.
This application is based on patent application No. 2003-294340 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
Centrifugal compressors used in turbo chillers exhibit unstable behavior called rotating stall and surging (stall) under low-airflow, high-head operating conditions.
In order to overcome this problem, conventionally, a portion of the hot gas exhausted from the centrifugal compressor is returned to the suction side of the centrifugal compressor to perform hot gas bypass control for maintaining the airflow (volumetric flow). (See, for example, Japanese Unexamined Patent Application, First Publication No. HEI 8-233382, paragraph [0005] and FIG. 4 therein.)
Before carrying out control for protecting the compressor, such as hot gas bypass control and so forth, it is necessary to predict or detect rotating stall and surging.
As one means for detecting surging, the technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-505525 (pages 7 to 9 and FIG. 1 therein), which is described below, is known.
In Japanese Unexamined Patent Application, First Publication No. 2000-505525, a technique for detecting surging based on a change in the electrical current of a variable-speed driving apparatus for driving the compressor and on a pressure difference between the two ends of the compressor (more concretely, the pressure difference between the condenser and the evaporator) is disclosed.
However, with the technique disclosed in the above-described Japanese Unexamined Patent Application, First Publication No. 2000-505525, there is a problem in that it is not possible to detect rotating stall or surging with high accuracy for the reason described below.
In the compressor of the turbo chiller, the condenser and the evaporator, which have large capacities compared to the compressor, are connected. Therefore, when detecting rotating stall or surging based on the pressure difference between the two ends of the compressor, even when changes in pressure occur as a result of the rotating stall and surging, the amount of change in the pressure difference between the two ends of the compressor becomes smaller because the pressure changes are absorbed in the high-capacity condenser and evaporator, and there occurs some time delay. Therefore, it is not possible to accurately determine the occurrence of rotating stall or surging.
Furthermore, detecting surging by combining the pressure inside the condenser and the temperature of cooling water that cools the condenser is also known. However, because the temperature of the cooling water changes suddenly depending on the number of operating cooling towers, which cool the cooling water outside, changes in the pressure inside the condenser are caused regardless of whether or not surging occurs. Under such circumstances, it is not possible to distinguish between changes in pressure as a result of changes in the cooling water temperature and changes in pressure as a result of surging.